A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling conduction periods of power switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of a power switch of the power converter. The duty cycle “D” is a ratio represented by a conduction period of a power switch to a switching period thereof. Thus, if a power switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50 percent). Additionally, as the voltage or the current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on the microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the power switches therein to maintain an output characteristic such as an output voltage at a desired value.
Power converters designed to operate at low power levels typically employ a flyback power train topology to achieve low manufacturing cost. A power converter with a low power rating designed to convert an ac mains voltage to a regulated dc output voltage to power an electronic load such as a printer, modem, or personal computer is generally referred to as a “power adapter” or an “ac adapter.” Some power adapters may be required to provide short-term peaks of power that are much greater than a nominal operating power level. A power adapter with a nominal 25 watt output power rating may be required to produce 60 watts of peak output power for a relatively small fraction of an operational cycle of the load, for example, for 40 milliseconds (“ms”) out of a 240 millisecond operational cycle of the load.
A component of the magnetic flux in a magnetic device, such as a power transformer (also referred to as a “transformer”), in certain power train topologies employed in a power converter is proportional to a peak operating current in a primary winding thereof. Accordingly, the magnetic device in power adapters should be sized for the peak power, rather than the nominal output power rating. However, oversizing the magnetic device increases its cost, which is an important consideration for high volume markets such as the markets for printers, modems, and personal computers. Designing a power converter for peak power also increases power losses at lower power levels because the power converter is typically designed to enter a discontinuous conduction mode (“DCM”) at a higher output power level than a power converter designed to operate only at a nominal output power level.
Power conversion efficiency of power adapters has become a significant marketing criterion, particularly since the publication of recent U.S. Energy Star specifications that require a power conversion efficiency of power adapters for personal computers to be at least 50 percent at output power levels below about one watt. The “One Watt Initiative” of the International Energy Agency is another energy saving initiative to reduce appliance standby power to one watt or less. These efficiency requirements at very low output power levels were established in view of the typical load presented by a printer in an idle or sleep mode, which is an operational state for a large fraction of the time for such devices in a home or office environment. A challenge for a power adapter designer is to provide a high level of power conversion efficiency over a wide range of output power.
Numerous strategies have been developed to reduce manufacturing costs and increase power conversion efficiency of power adapters over a wide range of output power levels including the incorporation of a burst operating mode at very low output power levels, the inclusion of an energy-recovery snubber circuit or a custom integrated controller, and a carefully tailored specification. Each of these approaches, however, provides a cost or efficiency limitation that often fails to distinguish a particular vendor in the marketplace. Accordingly, what is needed in the art is a design approach for a power adapter that enables a further reduction in manufacturing cost and improvement in power conversion efficiency that does not compromise end-product performance, and that can be advantageously adapted to high-volume manufacturing techniques. Additionally, what is needed in the art is a magnetic device employable with a power adapter or the like that enables the magnetizing inductance of the magnetic device to increase at lower current levels.